PROJECT SUMMARY Cigarette smoking is responsible for over 480,000 deaths annually in the US?three times higher than drug overdose- and alcohol-related deaths combined?making cigarettes the most deadly drug of abuse. Although smoking rates have declined over the past 50 years, tobacco use remains a critical problem. Depression symptoms, which are highly prevalent among smokers making a quit attempt, reduce the odds of cessation by 50% and are not addressed in standard interventions. New methods for addressing depressive symptoms in the context of a quit attempt are greatly needed. Behavioral activation therapy for depression (BAT-D) is a novel addition to standard smoking cessation interventions that can address depressive symptoms as a risk factor for treatment failure. Delivery through a mobile health application (mHealth app) makes this targeted treatment available in a high-reach format, as apps for smoking cessation are downloaded over 1.6 million times per year in the US alone. We conducted focus groups and individual user interviews to develop a limited prototype of a BAT-D mood management app called Actify. We then evaluated it in a single-arm pilot trial, which showed high engagement with the app (mean number of app logins=20), a promising short-term quit rate of 31%, and a statistically and clinically significant reduction in depression symptoms. In this study, we propose to refine the app based on single-arm pilot data, integrate smoking cessation content, conduct usability testing, and evaluate the acceptability and efficacy of the completed app in a pilot randomized controlled trial. In the first phase of the study, we will complete the development of Actify by adding cessation content, refining the app structure to improve usability, and conducting additional prototype testing with a small group of smokers (n=15). The second phase will be a pilot, randomized, controlled trial (n=240) comparing Actify with QuitGuide, the National Cancer Institute?s mHealth app for smoking cessation, on (a) acceptability (user satisfaction and number of app logins), (b) smoking abstinence (primary endpoint of 30-day point prevalence abstinence at 8 weeks post-randomization), and (c) mechanisms of change (depressive symptoms and change in behavioral activation). This study is significant in that addresses the important public health problem of low quit rates and resultant tobacco-related health disparities among smokers experiencing mood symptoms, and it builds off of a growing body of literature supporting the value of behavioral activation as a component of cessation counseling. It is innovative in several key respects, including: (1) Actify is the first standalone mHealth intervention of any kind to target depression and smoking cessation simultaneously; (2) it is the first pilot trial of a standalone mHealth app to address depression as a barrier to quitting smoking; and, (3) mHealth apps are a novel and promising addition to the larger body of mHealth research on text messaging, which has demonstrated efficacy. This project will generate pilot data to support an R01-level project testing the effectiveness of Actify in a fully-powered, randomized controlled trial.